[en] A three-dimensional, non-linear, primitive equation model with turbulent closure developed at the GeoHydrodynamics and Environment Research Laboratory (GHER), University of Liège, is designed to define marine weather-like processes ranging from mesoscale tides and storm surges, to synoptic frontal and eddy structures, to macroscale slowly-varying currents characteristic of the so-called “general circulation”. The model is applied here to the study of the hydrodynamics of the western Mediterranean as both a metagnostic (i.e. system-oriented) model and a diagnostic (i.e. process-oriented) model with both models allowed to run simultaneously and interactively. Critical processes such as deep water formation and the instabilities of the Algerian Current, are studied to provide a basic understanding of the physics needed to adequately parameterize sub-grid scale processes in the basin wide metagnostic model. This general circulation model provides boundary conditions for the finer grid diagnostic studies as well as the general overview of the basin's hydrodynamics. Further improvement is achieved by concurrent use of a variational inverse model to provide initial data fields and boundary conditions for the direct model. In turn, the metagnostic model provides hydrodynamic constraints which are imposed to the variational principle to ensure an interpolation/extrapolation of data compatible with the system's hydrodynamics. Results viz (i) a diagnostic study of the Algerian Current, (ii) a metagnostic study of the seasonal variability of the western Mediterranean Sea, are presented and compared with observations and with results of simpler (quasi-geostrophic, reduced gravity, etc.) models. Possible improved hydrodynamic forecasts of seasonal and interannual variability with the 3D primitive equation model is discussed with particular attention to the forecast's sensitivity to initial and boundary conditions.